Windows XP Corporate Professional - a key is provided so they don't need to contact Microsoft every time they install the OS on a system. This is handy in situations where there may be multiple installations over a network or frequent re-installations (as in a QA lab). Imagine if you upgraded 1000 machines to XP and had to contact Microsoft for EACH one. It would be a nightmare for all!

Windows XP Professional. - requires a contact with Microsoft to activate the OS. This is used in small business type situations where one contact will be made and (supposedly) never again.